Small rapidly rotating blades or rotary cutters have long been carried by the spindle of a milling machine and used to mill various patterns or make desired cuts either on stationary or sliding workpieces, or else on a symmetrical workpiece such as might be simultaneously rotated by a lathe relative to the cutter. As a more specialized usage, by employment of a helical guide -- applied either to the moving workpiece or to the axially progressing cutter -- such assembly was utilized in the past to cut screw threads. Such thread pattern obviously had to be formed on the inner or outer surface of a cylindircal bore or projection of the workpiece. If the remainder or bulk of such workpiece was too large or unwieldy to be rotated, it remained at rest and all handling or manipulation was done by the cutter assembly. Such procedure is necessary, for example, in threading the necks of turbine housings or large reaction vessels which it is not practical to rotate. When the cutter unit then has to assume almost any possible position, it no longer becomes feasible to power it from the spindle of a stationary milling machine. Accordingly the cutter assembly is then given its own drive motor and in effect is made completely portable. In addition, a distal segment of the cutter spindle is made adjustably displaceable radially so as to contact work surfaces of different diameters, without cessation of the rotary drive of the operating spindle. However, guide means are still required to plot or set the helical path followed by the cutter in forming screw threads; that is, every difference in pitch requires guidance along a different track or path. By the prior art, this was effected by use of a pair of threadedly-engaging guide tubes (one of which remained stationary and the other moved spirally therein) such pairs being inserted from time to time in the assembly to provide a pattern which the cutter then transferred, in effect, to the workpiece. To cut threads of a different pitch, such pair of guide tubes had to be removed from the assembly and another pair of a different pitch substituted in their place. Such procedure could be tedious and time consuming, and always required a supply of the preformed pairs. Such a milling cutter is shown in U.S. Pat. No. 3,762,272.